26 CONTEMPORARY SCIENCE 



there should be any very great difference in stability be- 

 tween rings having 7, 8 or 9 electrons. On the other hand, 

 we readily see that the geometrical symmetry of the ar- 

 rangement of the 8 electrons at (or rotating about) the 

 8 corners of a cube would not only account for a high 

 degree of stability but for the fact that an arrangement of 

 7 or 9 electrons would have no such stability. Chemical 

 considerations and Born and Lande's work on compressi- 

 bility also lead us to this spatial arrangement of the elec- 

 trons. We shall refer to the stable group of 8 electrons 

 by the term octet. From the principles already enunci- 

 ated it is clear that in the atoms of all the elements above 

 neon the inner electrons are arranged in the same way as 

 those of neon. 



From the atomic numbers of the inert gases we are thus 

 able to determine the number of electrons in the various 

 layers or shells of electrons which exist in the atoms. The 

 results are summarized in Table I. 



TABLE I DISTRIBUTION OF ELECTRONS IN THE VARIOUS SHELLS 



Inert Gas 



Number of Corresponding to 



Shell Electrons Completed Layer 



1st shell 2-2X12 He 2 



2nd shell, 1st layer 8 = 2X2 2 Ne 10 



2nd shell, 2nd layer 8 = 2 X 2 2 Ar 18 



3rd shell, 1st layer 18 = 2X3 2 Kr 36 



3rd shell, 2nd layer 18 = 2 X 3 2 Xe 54 



4th shell, 1st layer 32 = 2X4 2 Nt 86 



Thus the xenon atom with an atomic number 54 contains 

 54 electrons arranged as follows : Close to the nucleus 

 are two electrons which constitute the first shell. This 

 is surrounded by the second shell which contains two 

 "layers" of 8 electrons each. The third shell, which in 

 the xenon atom is the outside shell, contains 18 electrons. 



An examination of the number of electrons in the layers 

 (Table I, 2nd column) shows that they bear a simple 



